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Mercury, long thought to be a planet without the ingredients necessary for volcanism, was volcanically active as recently as one billion years ago.

Scientists learned, as a result of a 2008 discovery by the MESSENGER spacecraft of pyroclastic ash deposits on the planet’s surface, that the solar system’s smallest planet has experienced volcanism.

The general assumption has been that Mercury’s volcanic activity must have occurred early in the planet’s history, about 4 billion years ago.

Newly-published research indicates that some of the pyroclastic ash deposits seen by MESSENGER were actually deposited between about 1 and 3.5 billion years ago.

That means volatile compounds, which drive the explosive eruption of volcanoes, are likely more prevalent on Mercury than has been thought.

“Mercury, contrary to predictions, is not deficient in volatiles, but instead has an abundance of them, “ Sean C. Solomon, the director of the Lamont-Doherty Earth Observatory at Columbia University and MESSENGER’s principal investigator, said.

Using data obtained by cameras and spectrometers on board MESSENGER, a team of scientists examined 51 sites at which the ash was deposited. They observed that the vents through which the ash was expelled onto Mercury’s surface exhibited varying degrees of erosion and determined the age of the craters in which the ash exists.

The difference in the erosion of the vents shows that ash was deposited on the surface of the planet at a variety of times during its history.

“If [the explosions] happened over a brief period and then stopped, you’d expect all the vents to be degraded by approximately the same amount,” Timothy Goudge, a geology graduate student at Brown University and the lead author of the paper, said. “We don’t see that; we see different degradation states. So the eruptions appear to have been taking place over an appreciable period of Mercury’s history.”

The fact that some of the craters are older than the ash within them indicates that the ash must have been deposited in those craters after the impacts that produced the craters.

“These ages tell us that Mercury didn’t de-gas all of its volatiles very early,” Goudge said. “It kept some of its volatiles around to more recent geological times.”

Mercury’s large iron core has led to speculation that the planet may once have been significantly larger than it is now.

Solomon explained that scientists have tended to believe either that some of its mass was either burned away by the heat of the nearby Sun or blasted away during a collision with another celestial object. If either of those events had occurred, most, if not all, of Mercury’s volatile compounds would have been eliminated early in its history.

The new study’s confirmation that volatile compounds existed on Mercury until as recently as about 1 billion years ago casts doubt on those leading hypotheses about the planet’s formation.

“It really does tie the geochemistry and the geology together, pointing toward an origin we didn’t expect,” Solomon said.

Pyroclastic ash is produced by a volcano when it erupts. Volatile compounds in magma, such as carbon dioxide, hydrogen sulfide, sulfur dioxide, and water, change state from liquid to gas as the temperature of the magma rises and then expand, causing an increase in pressure that leads to the eruption.

The new paper appears in the March 28 edition of Journal of Geophysical Research: Planets.

MESSENGER was launched Aug. 3, 2004. Having completed its third year in orbit around Mercury last month, the probe is scheduled to crash into Mercury’s surface in March 2015.

The European Space Agency and the Japan Aerospace Exploration Agency plan a joint mission to Mercury to be launched in 2016.

Three independent lines of evidence support this conclusion: the first measurements of excess hydrogen at Mercury’s north pole with MESSENGER’s Neutron Spectrometer, the first measurements of the reflectance of Mercury’s polar deposits at near-infrared wavelengths with the Mercury Laser Altimeter (MLA), and the first detailed models of the surface and near-surface temperatures of Mercury’s north polar regions that utilize the actual topography of Mercury’s surface measured by the MLA.

The discovery may seem counter-intuitive, given Mercury’s high daytime surface temperatures. Though the thermometer can reach 800 degrees Fahrenheit on the side of the planet facing the sun, the planet’s north pole is permanently shielded from the nearby star.

Ice can be visually observed in the region between about the same latitude on Mercury (85 degrees North) as the Tropic of Cancer on Earth (90 degrees North) and the pole.

The probe has not confirmed evidence of ice at Mercury’s south pole.

MESSENGER, which launched in Aug. 2004, will experience orbital decay by 2015 as its fuel supply runs out. During its close approach to Mercury’s surface it may be possible to make more detailed observations of frozen water on our solar system’s inner-most planet.

It is difficult to choose the most fantastic space-related photograph of the year. There are, after all, many contenders. We had, for example, many “first-time” pictures of features on the Red Planet, our neighbor Mars. This one shows the west rim of huge Endeavor crater, which has a diameter of about 14 miles. The crater is characterized by geological features that are older than any others Opportunity has investigated during its mission.

Photo courtesy NASA/JPL-Caltech/Cornell/ASU

Then there was the first image ever obtained of the huge asteroid known as Vesta. NASA’s Dawn mission is in the midst of an exploration of the asteroid belt. This full view of Vesta was taken from a distance of 5,200 kilometers. The asteroid has a mean diameter of about 530 kilometers and is the second-largest object in the asteroid belt, second in size only to the dwarf planet Ceres.

Photo courtesy NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Sticking with the space theme, there are many other amazing images to see, starting with this dramatic one from the MESSENGER probe that shows just a small part of Mercury:

We cannot forget the incredible images obtained by the Hubble Space Telescope, either. This one, obtained by the telescope in Feb. 2011, shows a nebula (a region of space in which stars are being formed) that is about 2,000 light years away from Earth:

Photo courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

But my favorite picture from 2011 is this one, which was obtained by NASA’s Cassini probe. It shows a storm encircling the huge planet. The storm is the most intense, and the largest, ever observed by Cassini. It produces a significant amount of radio noise, which indicates that there is lightning occurring in Saturn’s atmosphere. One odd feature of storms on Saturn is that they occur when a huge amount of energy is released all at once, which is unlike the meteorological pattern on other planets.